In data display, optical data processing and other fields, recent attention is drawn to organic electroluminescent materials. Any of the electroluminescent materials emits light having a wavelength and an intensity characteristic of the material when it is sandwiched between electrodes and a voltage is applied thereto. It is believed that electrons and holes are injected from the respective electrodes into the organic electroluminescent material and moved in the organic electroluminescent material by the applied voltage, followed by recombination of the holes and electrons. The emitted light has a spectrum nearly identical with a fluorescence spectrum intrinsic for the electroluminescent material.
Organic and inorganic electroluminescent materials are known. The organic electroluminescent materials include blue electroluminescence-inducing anthracene known from old. The inorganic electroluminescent materials include well known semiconductors. Now, LED (light emitting diode) inducing red or yellow electroluminescence is drawing attention as a highly bright and stable electroluminescent material, among the electroluminescent semiconductors. LEDs inducing red, orange and green electroluminescences are combined to put into practical use as a three-color display for streets. Appl. Phys. Lett., Vol. 51, No. 12 (1987), pp 913-915 describes a two-layer electroluminescent device (EL device) prepared with the use of organic electroluminescent materials. This two-layer electroluminescent device is prepared by successively forming on an electrode of ITO (indium tin oxide) a hole injecting layer, an electroluminescent layer capable of transporting electrons and an electron injecting electrode of MgAg alloy according to vapor deposition. When a voltage of several tens of Volts is applied to this electroluminescent device, electrons and holes are injected into the electroluminescent layer to thereby emit light. With this two-layer electroluminescent device, the color of emitted light can be changed by choosing the type of the electroluminescent material. For example, a low molecular compound of an aluminum quinolinol complex (Alq3) is used as the electroluminescent material. Green electroluminescence is obtained by the use of the aluminum quinolinol complex as the electroluminescent material.
However, this two-layer electroluminescent device has a drawback in that as an electroluminescent layer the above low molecular compound deposited is crystallized to cause detachment of the organic layer (electroluminescent layer) from the electrode to thereby no longer emit light. There is another drawback that the two-layer electroluminescent device generates heat accompanied with the emission of light to markedly increase the temperature of the device, so that the device is deteriorated.
Saito et al. have proposed a three-layer organic electroluminescent (EL) device for improving electroluminescene efficiency (Jpn. J. Appl. Phys., Vol. 27, No. 2, 1988, pp. L269-L271 and Jpn. J. Appl. Phys., Vol. 27, No. 4, 1988, pp. L713-L715). This three-layer organic electroluminescent device is prepared by successively forming on an ITO electrode (positive electrode) a hole injecting layer, an electroluminescent layer, an electron injecting layer and an MgAg electrode (negative electrode). In this multi-layered structure, the hole injecting layer, the electroluminescent layer and the electron injecting layer are composed of organic layers. In this EL device, the injection efficiency of holes from the positive electrode into the electroluminescent layer is improved by the hole injecting layer, and the injection efficiency of electrons from the negative electrode into the electroluminescent layer is improved by the electron injecting layer. Consequently, the threshold voltage required for the electroluminescence of the three-layer organic electroluminescent device is only several Volts, and blue electroluminescence having been difficult with inorganic materials can be attained with this three-layer organic electroluminescent device at the threshold voltage comparable to that with the above two-layer organic electroluminescent device.
However, this three-layer organic electroluminescent device uses organic low molecular compounds, such as oxadiazoles each having a single oxadiazole ring, as materials capable of hole injection or electron transport. Most of these low molecular organic compounds have melting points as low as up to 300.degree. C. Therefore, disadvantageously, the three-layer organic electroluminescent device prepared with the use of the above low molecular organic compounds has poor heat resistance to thereby suffer from thermal degradation of properties, or recrystallization occurs in the device to cause degradation of the same.
It has been proposed to form at least one of the organic layers, which are electron injecting (transporting) layer, the electroluminescent layer and the hole injecting (transporting) layer with the use of a polymeric thin film for avoiding the above thermal degradation and recrystallization in the electroluminescent device. For example, in Japanese Patent Laid-Open Publication No. 2096/1992, a process for producing a polymeric thin film EL device is proposed in which a polymeric thin film comprising an electroluminescent low molecular weight material or a low molecular weight material capable of hole injection and electron transport is formed by a wet process, such as spin coating or dip coating. Several tens of Volts must be applied to the thus obtained for providing effective electroluminescent brightness. Even if this high voltage is applied, the electroluminescent brightness is only up to 200 cd/m.sup.2.
In the formation of the above polymeric thin film by spin coating, there is a drawback such that pin holes are likely to form in the preparation of polymeric thin film, pin holes which cause the device to break during the driving of the device.
Further, the formation of the polymeric thin film according to the wet process is likely to contain impuirities into the device. The impurities accelerate the degradation of the device.
Although the formation of polymer layer by wet process is easy to make the device, the process is not suitable for making the device. Because when the electroluminescent device is produced by forming an organic layer (upper layer) on an organic layer (under layer) according to the wet process, it is requisite to select a solvent which does not dissolve or leach the organic under layer in the preparation of a coating fluid for forming the upper organic layer. In the formation of an organic layer (upper layer) on an organic layer (under layer) according to the wet process, the materials usable for forming the under layer and the upper layer and the solvents for dissolving the materials are limited. Consequently, there is a problem that the types of the polymeric materials capable of forming the organic layer of the polymeric thin-film electroluminescent device and the low molecular materials which can be contained in the polymeric materials are extremely limited.
On the other hand, in Japanese Patent Laid-Open Publication No. 274693/1992, it is proposed to use, as an electroluminescent layer or a charge injecting/transporting layer of an electroluminescent device, a thin film of a polyimide represented by the formula: ##STR2## wherein X and Y are as defined below, and n is an integer indicating the degree of polymerization, the polyimide being prepared through a polyamic acid (polyimide precursor) by reacting an acid dianhydride represented by the formula: ##STR3## wherein X represents an organic group including an aromatic, with a diamino compound represented by the formula: EQU H.sub.2 N--Y--NH.sub.2 (VIII)
wherein Y represents an organic group having EL function (at least one of charge injecting, charge transporting and electroluminescent functions) according to the vapor deposition polymerization process.
In the above polyimide of the formula (IX) indicates that the nitrogen atom forming sp.sup.3 hybrid orbital and the carbon atom of the carbonyl group constitute .sigma. bond, which forms part of the principal chain of the polyimide. This .sigma. bond part restricts the .pi. electron conjugation chain, so that excellent electron conductivity, thus high electroluminescence, cannot be expected from the electroluminescent device having a thin film of the polyimide as an electroluminescent layer or a charge injecting/transporting layer.
Further, the polyimide represented by the formula (IX) has a carbonyl group with a large dipole moment at the site of the imide bond. This carbonyl group is known to function as a trap of carriers (electrons and/or holes), thereby lowering the mobility of carriers. High electron conductivity cannot be expected from the polymeric thin film having low carrier mobility.